The present invention relates to lubricating oil compositions, such as power transmission fluids which contain an improved additive, effective as a corrosion inhibitor, oxidation inhibitor and/or friction modifier as well as to a process for preparing such additive. More specifically, the present invention relates to amine or ammonium salts of mercaptobenzothiazole (MBI) solubilized with aromatic carboxylates as lubricating oil composition compatible additives.
Mercaptobenzothiazole has been used in power transmission shift fluids, such as automatic transmission fluid compositions, as a corrosion inhibitor. However, difficulties have been encountered in effectively solubilizing the material into the composition. Attempts have been made to use special solvents and blending techniques to deal with the compatibility problem. Thus, materials such as hexyl phthalate have been used as special solvents for the mercaptobenzothiazole additive. Such solvent systems, however, are still associated with compatibility problems. For example, while a 1:1 molar ratio mixture of mercaptobenzothiazole and dihexylphthalate results in solubilization of the mercaptobenzothiazole at 127.degree. C., the mercaptobenzothiazole precipitates out of solution below this temperature. Consequently, such solutions cannot be effectively utilized for automatic transmission packages which are blended at temperatures of about 65.degree. C.
Commonly assigned allowed U.S. patent application Ser. No. 573,122, filed Jan. 23, 1984, by Ryer, J., Bloch, R., and Deen, H., discloses the concept of employing a molar excess of amine (i.e., 1.2:1 and higher) to form the mercaptobenzothiazole salt with the excess amine acting to assist in solubilizing the salt. While this technique is advantageous, it would be of significant economic benefit if one could reduce the amount of amine employed because of its high cost, and yet still be capable of compatibly solubilizing the mercaptobenzothiazole in lubricating oil compositions. It is toward this goal that the present invention is directed.
By way of background, there are two general types of reactions of amines with mercaptobenzothiazole, namely, (1) the oxidative condensation of the amine with mercaptobenzothiazole to yield a sulfenamide, and (2) the formation of amine salts from mercaptobenzothiazole and the amine. Such reactions are distinct and the former does not pertain to the present invention. Moreover, neither type of reaction is known for use with the specific class of ester substituted benzenes in accordance with the present invention.
For example, U.S. Pat. No. 3,600,398 discloses the sulfenamide forming reaction in the presence of an oxidizing agent. A large excess of amine is disclosed to be necessary to prevent formation of the disulfide, e.g., at least 8:1 (Col. 2, Lines 55 et seq.). In addition, water is a required solvent although tetrahydrofuran in certain instances is a suitable alternative solvent, provided a ten-fold excess of amine is employed.
U.S. Pat. No. 4,258,197 also discloses a sulfenamide forming reaction in the presence of water or organic solvent. However, the only organic solvent disclosed is butyl cellosolve. When an organic solvent is used, the molar ratio of mercaptobenzothiazole to amine is 1:1.5 to 1:2. Moreover, even in the absence of the use of water as a solvent, at least 10% by weight of the reactants must be water to initiate the reaction.
U.S. Pat. No. 3,966,623 discloses a synergistic corrosion inhibiting combination of a specific thiadiazole and the amine salt of mercaptobenzothiazole for lubricating oil compositions. The method of preparing the mercaptobenzothiazole amine salt is not disclosed.
U.S. Pat. No. 3,539,512 discloses a grease composition which contains, in addition to five other required ingredients, a high molecular weight amine salt of mercaptobenzothiazole that functions as an anti-rust agent. The amine salts are disclosed as being prepared by reacting equivalent amounts of mercaptobenzothiazole and amine, generally at elevated temperatures (Col. 3, Line 37). However, it will be observed at Col. 4 that the grease formulation contains a polymer in oil dispersion, and the grease is generally milled. Obviously, the compatibility requirements of the mercaptobenzothiazole salt in grease are substantially different than in lubricating oil compositions. Moreover, no solvent at all is disclosed.
U.S. Pat. No. 2,437,170 discloses the use of the reaction product to mercaptobenzothiazole and dodecylamine as a fungicide, insecticide, mildew proofing agent, rot proofing agent, disinfectant, germicide, and an anti-fouling agent, for marine paints and for analogous purposes. The mercaptobenzothiazole and amine are reacted at a molar ratio thereof of 1:0.91. The end product of the reaction is a dry powder which can be dissolved in a suitable solvent. The disclosed solvents are xylol, toluol, butyl alcohol, ethyl alcohol, diacetone alcohol, and isophorone. It is also disclosed that the reaction may be carried out in the presence of a solvent or non-solvent vehicle or both. No specific solvents are disclosed for this purpose and one must infer that the alcohols or ketones disclosed above define such solvents. The environments in which the mercaptobenzothiazole salt is employed are disclosed as being paints, enamels, lacqueres, as well as solutions of the salt alone for mildew proofing of cellulosic materials. Consequently, compatability in lubricating all compositions is not a consideration either from the standpoint the problems associated therewith or in relation to the solution to such problems.